In a propaganda video released by the SITE Intelligence Group Wednesday, an ISIS member describes the Barcelona perpetrators as "our brothers," while another threatens "Spanish Christians" and promises to return the country to the "land of the Caliphate."

Soon after the Barcelona attack, ISIS' media wing, Amaq, issued a statement calling the attackers "soldiers of the Islamic State," although ISIS did not explicitly claim responsibility.

Four men have been arrested since the attack -- one in Alcanar and three in Ripoll. Three were Moroccan citizens and another was Spanish; the men ranged in age from 21 to 34. None were on authorities' radar for potential terrorism links.

Terror cell base

Evidence of the planned attacks has alarmed many in Spain, highlighting the possibility the authorities knew nothing of an advanced plot to mount a terror attack until an accidental explosion at the perpetrators' base.

A subsequent investigation has focused on that base -- a property in Alcanar, a coastal village southwest of Barcelona, that blew up last week before the attacks.

The debris of a house in the village of Alcanar, Catalonia, is seen Thursday after it collapsed due to an explosion.

A source briefed on the probe told CNN a preliminary assessment indicated traces of the powerful explosive TATP in the rubble. TATP was used in a series of deadly bombing attacks in Europe in recent years.

The loss of those explosives meant the subsequent attack in Barcelona was "more rudimentary than they originally planned," Catalan police chief Josep Lluis Trapero said.

Police believe Muslim cleric Abdelbakir El-Satty is among those who died in the Alcanar blast. Some Spanish media reported he shared a jail cell with one of the terrorists involved in the 2004 Madrid bombings that killed at least 191 people and left hundreds injured.

Responding to those reports, Catalan Justice Minister Carles Mundó i Blanch said "we are not aware that this person had any communication with any inmate in any prison in Catalonia. He finished his sentence and went free."

How the attacks unfolded

The first sign of the terror cell's activities -- although its significance was not immediately understood -- came the night before the Barcelona attack when the house in Alcanar was almost completely destroyed by an unexpected explosion.

Thwarted by the explosion of ammunition and possibly alarmed at the attention it had drawn, it appears that the surviving members of the cell scrambled to mount an alternative plan.

The next day at about 5 p.m., a rental van was driven at high speed from Barcelona's Plaça de Catalunya deep into the teeming crowds on the city's most famous thoroughfare, Las Ramblas.

At least 13 people were killed and 120 injured.

Hours later, as details of the atrocity in Barcelona were still emerging, five attackers drove an Audi A3 into several pedestrians in Cambrils, 120 kilometers (75 miles) south of Barcelona, early on Friday morning.

Police engaged in a shootout with the attackers, and all five were shot dead, four of them by one officer.